The operation of a PM wind turbine generator is highly dependent on the condition of the permanent magnets. Faults such as broken magnets or non-uniformly magnetization levels of the magnets give rise to a degraded quality and utilization of the generator.
It is well known to assemble an electrical PM machine with pre-magnetized magnets in prepared slots comprised in the rotor as disclosed in Danish patent DK 172430.
A disadvantage by the described method is that at the installation extensive magnetic forces act between the magnets and other magnetic components of the electrical machine. This requires special precautions to be taken during installation comprising special tools handling the magnets, magnetic components and the forces that interact.
Furthermore as the magnets are magnetized prior to installation, the condition of the magnets after installation is left uncertain until the wind turbine is operated.
US patent application US 2002/0149275 discloses an electrical machine comprising magnetic-field sensors mounted on the stator as to detect the angular position of the rotor.
It is an object of the invention to provide technique without the above mentioned disadvantages and especially it is an object to provide technique that enables monitoring the condition of the permanent magnets after they are installed in the rotor of a wind turbine generator.